Lithium-ion battery packs in hybrid electric vehicles (HEVs) and fully electric vehicles (EVs) in the near term will contain carbon-based active materials in the anode. However, full market penetration will require further non-incremental improvements in cyclic capacity at lower costs. Typical state of the art lithium battery anodes are composites of 90% (by mass) graphitic carbon and 10% polymeric binder coated onto metallic copper current collectors.
Previous work at the Oak Ridge National Laboratory (ORNL) has shown that good performance can be achieved on the cathode side by replacing the binder and current collector with highly conductive graphitic carbon fibers. In this work, particles of the cathode active material were coated directly onto the carbon fiber; the carbon fibers were the backbone of the electrode architecture and conduit for electron transport to the active material but did not participate in lithium intercalation. Attempts have been made to utilize these carbon fibers as the active material on the anode side, but low capacities were realized due to alignment of the basal planes of graphite crystallites parallel to the carbon fiber axis. The basal plane is effectively a barrier to lithium diffusion; lithium insertion is limited to defect sites in the plane. Researchers at the U.S. Army Research Laboratory recently presented results from their characterization of commercially available carbon fibers and related structures as anodes in lithium ion batteries. The best reversible electrochemical capacity was 158 mAh g−1, less than half the theoretical capacity of graphite. The authors note that carbon fibers for the composite industries typically consist of a disordered carbon core surrounded by a graphitic sheath, which may explain the low capacities obtained in the study.
The intercalation compound of lithium with graphite with a stoichiometry of LiC6 corresponds to a theoretical charge capacity of 372 mAh·g−1. It has been demonstrated that it is possible to surpass this capacity using several modifications of carbon and graphite, many of which do exceed the theoretical charge capacities. However, in many cases of high capacity carbons (hard carbons and disordered carbons), the stability upon cycling is limited. Activated carbons containing micropores (<2 nm) and no mesopores (2 to 50 nm) were shown to reversibly insert lithium electrochemically in non-aqueous salt solutions. Although activated carbons can be prepared with specific surface areas as high as 2500-3000 M2·g−1 by extensively developing their porosity, they usually possess a very wide pore size density of the material.
Carbon fibers are mixed ionic/electronic conductors that can have relatively high electrical conductivities >10-50 S/cm. The microstructure and graphitic content of carbon fibers are critical for effective insertion of lithium into carbon fibers; the microstructure should be controlled such that the graphene planes of graphite crystallites are oriented off-parallel to the fiber axis. Charge storage capacities in carbon fibers derived from mesophase pitch with a radial texture are comparable to those of graphite, but pitch-based fibers are expensive. Pyrolytic carbons from rice husks have been shown to have reversible capacities over 700 mAh g−1 for several hundred cycles; however, the additional processing steps required for binding the powder form graphite dominates its cost. Previous studies on the pyrolysis of epoxy for battery applications have shown that turbostratic disorder and crystallite size significantly increase the specific capacity from under 200 mAh g−1 to over 700 mAh g−1.